LilYellowZQ8
12-07-04, 08:51 PM
Written by Jedi Master.
The air/fuel ratio is a common source of confusion to many people. It refers to the ratio of air to fuel exiting from the engine, and it is a good measure (in combination with an exhaust gas temperature gauge) of measuring engine tune. The air/fuel ratio is commonly represented as a single numeral as in 14.7. This actually represents the number of air particles exiting per single fuel molecule (ie., 14.7:1). The 14.7:1 just mentioned is actually what is called stoichiometric - or the ideal air/fuel ratio, at which temperatures are controlled and fuel economy is optimized. Typically cars make best power at about 12.5:1 air/fuel ratio, but this is harder on the emissions parts and sensors, which reduces lifetime.
As mentioned before, exhaust gas temperature, in combination with air/fuel ratio, is a good measure of the state of engine tune. This is because the air/fuel ratio affects the heat generated during the combustion phase of the engine cycle. Fuel acts as a cooling agent for the cycle, and thus having a numerically lower air/fuel ratio - which is considered to be a RICHER air/fuel ratio (ie., richer in the amount of gas exiting) will result in lower exhaust gas temperatures. Similarly, leaning out the mixture by injecting less gasoline causes the overall exhaust gas temperatures (commonly referred to as EGTs) to rise. Frequently a change from a lean to a rich mixture can cool the exhaust charge by as much as 400 degrees Fahreinheit, so this is a very practical matter.
That being said, it's often very difficult to determine the air/fuel ratio an engine is generating, and measuring exhaust gas temperature is problematic as well. For example, where does one place the exhaust gas temperature probe? At what distance from the exhaust ports? Should it be placed after the merge collector of the exhaust manifold or headers, or should it be placed nearer a particular cylinder? Will this result in biased readings if for any reason a single cylinder is running at higher temperatures than any other? Problems like this exist also for the air/fuel ratio. It is a constantly changing variable which your onboard computer attempts to control with some degree of accuracy.
Measuring the air/fuel ratio is problematic. Because it exists not as a physically static constant but as a ratio, the sensors that your car uses to determine it (commonly referred to as O2 sensors) are ineffectual at best. The sensors used are, in older cars, of a non-heated single-wire type, which use the amount of air and fuel in the exhaust to vary the resistance of the surface of the sensor as the hot exhaust charge passes over it (in relation to the outside air), while newer cars typically use a four-wire heated sensor. Obviously the reaction to changes in the air/fuel ratio is not immediate, and thus large lags in the detection of a change in the air/fuel ratio happen frequently. In addition, these sensors must be mass-produced cost-effectively, and as such are engineered to work only in a small temperature band. If the engine is operating in a temperature outside of the optimal band, readings can be very sluggish and delayed by several seconds, or worse, completely wrong, as the membrane on the outside of the O2 sensor is temperature-dependent. This problem is exaggerated in the single-wire type, as they rely on the engine to bring them up to operating temperatures, and during that time period the sensor is completely ineffectual. Even worse than that, the O2 sensors present on your car tend to be effective only in a very narrow air/fuel ratio band, and their responsiveness is directly proportional to their age - O2 sensors wear out and get contaminated/fouled (even by handling!), and especially when asked to monitor a continually rich condition. This isn't always true, as there are O2 sensors available which do not rely on temperature to drive the metering and have very rapid response times (these are called wideband O2 sensors, where the wideband part refers to the band of air/fuel ratios in which the sensor is accurate - stock sensors are narrowband). Unfortunately, these wideband O2 sensors typically cost several thousand dollars US!, and actually using them would cost several thousand more for custom installations, but the usage of these sensors can be purchased at dynograph facilities during tuning sessions.
As mentioned before, the O2 sensors themselves operate by the particles in the hot exhaust gas modifying the resistance of the membrane surrounding the O2 sensor in relation to the outside ambient air. The onboard computer in your car (commonly referred to as an ECU, or Electronic Control Unit) can measure this resistance variation on the fly by passing a voltage through the wire (usually between 0 and 1 volts) and measuring what comes back, though this is pretty much unimportant - all that matters is that the computer can do it.
The computer acts to vary the air/fuel ratio by taking into account engine RPM, throttle position, engine temperature, timing advancement, and a number of other factors. As the engine cycles/turns, the computer calculates the optimal period for the fuel injectors to open and spray fuel into the engine (remember that fuel is pressurized, so that the time the injector is open is directly proportional to how much fuel gets sprayed in - ie., fuel spray happens at an effectly constant rate, so that having an injector open for 0.05sec will spray less fuel than 0.10sec). Obviously, the air/fuel ratio is proportional to how much fuel is injected, and therefore the computer uses the determined air/fuel ratio continually to tune and optimize its fuel injection maps. This is what causes the 'bouncing' or 'scrolling' commonly observed in air/fuel gauges at part throttle - the computer seeks to keep the engine operating at the optimal air/fuel ratio by richening and leaning the fuel mixture, attempting to get the mix *just right*. However, there will always be a certain set amount of 'bounce' no matter how long the computer seeks to tune the mixture, as it is also used to test that the O2 sensor is still functioning.
The air/fuel ratio is a common source of confusion to many people. It refers to the ratio of air to fuel exiting from the engine, and it is a good measure (in combination with an exhaust gas temperature gauge) of measuring engine tune. The air/fuel ratio is commonly represented as a single numeral as in 14.7. This actually represents the number of air particles exiting per single fuel molecule (ie., 14.7:1). The 14.7:1 just mentioned is actually what is called stoichiometric - or the ideal air/fuel ratio, at which temperatures are controlled and fuel economy is optimized. Typically cars make best power at about 12.5:1 air/fuel ratio, but this is harder on the emissions parts and sensors, which reduces lifetime.
As mentioned before, exhaust gas temperature, in combination with air/fuel ratio, is a good measure of the state of engine tune. This is because the air/fuel ratio affects the heat generated during the combustion phase of the engine cycle. Fuel acts as a cooling agent for the cycle, and thus having a numerically lower air/fuel ratio - which is considered to be a RICHER air/fuel ratio (ie., richer in the amount of gas exiting) will result in lower exhaust gas temperatures. Similarly, leaning out the mixture by injecting less gasoline causes the overall exhaust gas temperatures (commonly referred to as EGTs) to rise. Frequently a change from a lean to a rich mixture can cool the exhaust charge by as much as 400 degrees Fahreinheit, so this is a very practical matter.
That being said, it's often very difficult to determine the air/fuel ratio an engine is generating, and measuring exhaust gas temperature is problematic as well. For example, where does one place the exhaust gas temperature probe? At what distance from the exhaust ports? Should it be placed after the merge collector of the exhaust manifold or headers, or should it be placed nearer a particular cylinder? Will this result in biased readings if for any reason a single cylinder is running at higher temperatures than any other? Problems like this exist also for the air/fuel ratio. It is a constantly changing variable which your onboard computer attempts to control with some degree of accuracy.
Measuring the air/fuel ratio is problematic. Because it exists not as a physically static constant but as a ratio, the sensors that your car uses to determine it (commonly referred to as O2 sensors) are ineffectual at best. The sensors used are, in older cars, of a non-heated single-wire type, which use the amount of air and fuel in the exhaust to vary the resistance of the surface of the sensor as the hot exhaust charge passes over it (in relation to the outside air), while newer cars typically use a four-wire heated sensor. Obviously the reaction to changes in the air/fuel ratio is not immediate, and thus large lags in the detection of a change in the air/fuel ratio happen frequently. In addition, these sensors must be mass-produced cost-effectively, and as such are engineered to work only in a small temperature band. If the engine is operating in a temperature outside of the optimal band, readings can be very sluggish and delayed by several seconds, or worse, completely wrong, as the membrane on the outside of the O2 sensor is temperature-dependent. This problem is exaggerated in the single-wire type, as they rely on the engine to bring them up to operating temperatures, and during that time period the sensor is completely ineffectual. Even worse than that, the O2 sensors present on your car tend to be effective only in a very narrow air/fuel ratio band, and their responsiveness is directly proportional to their age - O2 sensors wear out and get contaminated/fouled (even by handling!), and especially when asked to monitor a continually rich condition. This isn't always true, as there are O2 sensors available which do not rely on temperature to drive the metering and have very rapid response times (these are called wideband O2 sensors, where the wideband part refers to the band of air/fuel ratios in which the sensor is accurate - stock sensors are narrowband). Unfortunately, these wideband O2 sensors typically cost several thousand dollars US!, and actually using them would cost several thousand more for custom installations, but the usage of these sensors can be purchased at dynograph facilities during tuning sessions.
As mentioned before, the O2 sensors themselves operate by the particles in the hot exhaust gas modifying the resistance of the membrane surrounding the O2 sensor in relation to the outside ambient air. The onboard computer in your car (commonly referred to as an ECU, or Electronic Control Unit) can measure this resistance variation on the fly by passing a voltage through the wire (usually between 0 and 1 volts) and measuring what comes back, though this is pretty much unimportant - all that matters is that the computer can do it.
The computer acts to vary the air/fuel ratio by taking into account engine RPM, throttle position, engine temperature, timing advancement, and a number of other factors. As the engine cycles/turns, the computer calculates the optimal period for the fuel injectors to open and spray fuel into the engine (remember that fuel is pressurized, so that the time the injector is open is directly proportional to how much fuel gets sprayed in - ie., fuel spray happens at an effectly constant rate, so that having an injector open for 0.05sec will spray less fuel than 0.10sec). Obviously, the air/fuel ratio is proportional to how much fuel is injected, and therefore the computer uses the determined air/fuel ratio continually to tune and optimize its fuel injection maps. This is what causes the 'bouncing' or 'scrolling' commonly observed in air/fuel gauges at part throttle - the computer seeks to keep the engine operating at the optimal air/fuel ratio by richening and leaning the fuel mixture, attempting to get the mix *just right*. However, there will always be a certain set amount of 'bounce' no matter how long the computer seeks to tune the mixture, as it is also used to test that the O2 sensor is still functioning.